In a slip forming process, the concrete mix is extruded with the help of auger feeders into a mold or through nozzles, whereby the casting machine is propelled along the casting bed by the reaction forces imposed on the auger feeders. The ready-cast product remains on the casting bed.
A major problem in concrete products fabricated by slip-form casting is related to strength variations in the ready-cast product that are mainly caused by uneven compaction of concrete mix over the entire casting operation. As the casting machine moves propelled by the extrusion pressure exerted by the auger feeders, variations in the travel resistance and composition of concrete mix are reflected on the pressure generated by the auger feeders and, thereby, on the compaction of the concrete mix. Variations in the travel resistance are partly caused by changes in the weight of the casting machine due to variations in the amount of concrete mix in the feed hopper of the casting machine and partly due to changes in the travel resistance invoked by the pretensioning wires. Depending on the type of product, the resistance imposed by the pretensioning wires on the wire guides of the casting machine may vary widely due to the different number of wires used in various kinds of slabs. The number of wires in different slab types is determined by the design load-bearing capability and span of the slab. Respectively, the degree of compaction attained in a given concrete mix is affected by aggregate size distribution and proportion in the mix, as well as by the moisture content of the concrete mix.
In patent publication FI 97455 is disclosed a method wherein the degree of compaction in the concrete mix is sensed indirectly by measuring the input power to a top troweling beam and then the compaction movement or concrete mix feed rate is controlled. However, as such a casting machine travels propelled by the reaction force of the extrusion pressure alone, this method is incapable of fabricating a sufficiently tightly compacted product if the casting machine happens to run too easily at a low travel resistance.
In patent publication GB B 1 586 181 is disclosed an embodiment wherein the travel speed of the casting machine is kept maximally constant by controlling the input power or reaction force of the auger feeders, thus aiming to achieve a constant degree of compaction in all parts of the cast product. However, casting at a constant speed does not guarantee uniform compaction with varying qualities of the concrete mix.
In patent publication FI 84575 is disclosed a method wherein is measured the pressure imposed on the internal walls of the mold section of a casting machine. However, pressure measurement at the mold walls is not particularly well suited for controlling the compaction of concrete mix because even at a constant external pressure, a nonuniform degree of compaction may occur in a concrete mix due to its varying moisture content. The auger feeder only produces the extrusion pressure but does not actively contribute to the compaction of the concrete mix. All known methods are incapable of sensing the degree of compaction in a concrete mix by way of a direct measurement performed on the concrete mix during an on-going slip-form casting cycle.